A Japanese research team developed the "wireless power transmission method using two parallel wire lines," which can wirelessly supply electricity to a moving device, electric vehicle (EV), etc.

The research team consists of a laboratory headed by Minoru Okada, professor at Nara Institute of Science and Technology (NAIST), Graduate School of Information Science, and Daihen Corp.

The method is a kind of wire line-type wireless power transmission technology based on the magnetic resonance method. But it does not lay out many power transmission coils as power transmission devices and uses two parallel conductive lines. The ends of the lines can either be connected or unconnected.

This method realizes wireless power transmission capability because the research team found matching conditions under which impedance matching can be achieved for the entire system including load when a power reception coil approaches. Specifically, it proved that the parallel wire lines and the equivalent circuits of the antenna on the power reception side and load at the time of magnetic coupling are equivalent to a high-pass filter (HPF) having a specific impedance.

The HPF works as an impedance converter that matches the impedances of power supply and load.

The impedance matching conditions for a system with a power receiving device moving above the wire lines depend on the position of the power reception coil in addition to the diameter of and distance between the lines. Therefore, the research team expects to actively match the impedance of the system by using a matching box embedded in the power supply.

We interviewed Takeshi Higashino, associate professor at NAIST, who found the matching conditions, etc, on the potential of the new wireless power transmission system.

Q: Why did you develop the power transmission device?

Higashino: Conventional line-type wireless power transmission systems that lay out coils for power transmission are vulnerable to position aberration. So, we considered solving the problem by using parallel wire lines. Currently, they produce the standing wave of current density, and their "belly" and "knot" have different amplitudes, meaning different transmittable power. But we consider that this issue can be solved by, for example, making improvements to the shape of the wire lines.

Q: There have already been several line-type wireless power transmission technologies. How is your technology different from them?

Higashino: While a research group headed by Takashi Ohira, professor at the Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, uses electric field coupling (See related article), our technology uses magnetic coupling.

Wireless power transmission technologies using magnetic coupling are also being developed by a research group led by Toshio Ishizaki, professor at the Department of Electronics and Informatics, Ryukoku University, and Ryutech Corp, respectively. But the designs of their power transmission antennas are different from that of ours. And they use traveling waves, not standing waves.

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Q: How much power transmission efficiency does your technology realize?

Higashino: When we prototyped a toy rail, it was possible to transmit 40W of electricity with an efficiency of a few percent or higher. However, we did not control the impedance of the power reception coil. In theory, if the impedance of the power reception coil is controlled, the efficiency can be improved up to about 85%, even in consideration of the loss caused by the electrical resistance of the wire line and eddy current. Without the loss, the theoretical efficiency is 100%.

The position aberration of the power reception coil in the transverse direction can be tolerated up to about 2cm when the distance between the lines is 4cm.

Q: What did you use for the power supply? Does the power transmission efficiency include the efficiency of the power supply?

Higashino: The power transmission efficiency does not include the efficiency of the power supply. At this point, we use a 50Ω power supply (such as a class A device), and the efficiency of the power supply is 50% or lower. This is because we are currently focusing on the improvement of the efficiency of the wireless transmission part.

Q: How about the frequency?

Higashino: With the prototype and in a theoretical calculation, it was 13.56MHz. Basically, we expect it to range from several megahertz to several tens of megahertz. With a low frequency, a large power reception coil is required. But with a high frequency, a different issue arises.

Q: What kinds of applications do you expect?

Higashino: We expect that the parallel wire lines will be installed on a sidewalk along a roadway to charge EVs, electric busses and bicycles, pedestrians' smartphones and so forth. It is possible that our first goal will be to supply electricity to industrial robots at manufacturing plants.

Daihen, which cooperates with us in the development, has excellent impedance control technologies and was responsible for the development of the matching box. We are considering having a field test with a large-scale system in the near future.